A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The pattern to be transferred to the substrate may include very small structures. Any mechanical disturbance such as a vibration or the like may result in an erroneous transfer rendering the substrate unusable. To minimize a transfer of vibrations and other mechanical disturbances from any external structure, such as a support structure or floor, to the lithographic apparatus, in particular a substrate table, a pattern support structure and/or a projection system thereof, it is known to employ one or more vibration isolation support devices to support the lithographic apparatus.
Such a vibration isolation support device supports the lithographic apparatus, i.e. at least partly compensates a gravitational force exerted thereon. Thus, the vibration isolation support device provides a mechanical connection between the lithographic apparatus and an external structure, e.g. a floor. Depending on a frequency of the vibration, a vibration present in the external structure may be (partly) damped and/or attenuated by the isolation support device and/or (partly) transferred from the external structure to the lithographic apparatus. The amount of attenuation and/or transfer of a vibration of a certain frequency depends on a stiffness of the vibration isolation support device. A relatively small stiffness results in relatively high vibration isolation. Therefore, it is desirable to have a vibration isolation support device having a small stiffness.
A well-known vibration isolation support device is an airmount. Such an airmount includes a gas chamber containing an amount of pressurized air and a moveable member partly positioned in the gas chamber. The pressure of the air exerts a support force on the moveable member. An object such as a lithographic apparatus, or a part thereof, is supported on the moveable member. The known airmount has such a (positive) stiffness that it is suitable to attenuate vibrations above a predetermined frequency such that a pattern having a predetermined minimum feature size may be correctly transferred. However, there is a need to transfer smaller structures having smaller feature sizes. Therefore, it is desirable to minimize errors that may result from resonant responses of the support device.